1. Field of the Invention
The present invention relates to a surface-mount crystal oscillator, and more particularly to a surface-mount crystal oscillator which is amenable to miniaturization and which enables protection of an integrated circuit (IC) including, for example, an oscillation circuit.
2. Description of the Related Art
Surface-mount crystal oscillators, in which a quartz crystal unit and an oscillation circuit which works together with the crystal unit are accommodated in a surface-mount receptacle, are widely used as reference sources for frequency or time in various portable devices due to their light weight and small size. Surface-mount crystal oscillators can be divided between two types according to construction of receptacle: monolithic type and bonded type. In a monolithic-type crystal oscillator, a receptacle is employed in which are formed as a monolithic unit: a crystal unit housing which houses the crystal unit, and an element housing which houses circuit elements in which components such as an oscillation circuit are formed. In a bonded-type crystal oscillator, on the other hand, a first receptacle portion which includes a crystal unit housing and a second receptacle portion which includes an element housing are formed separately and then bonded together to construct the crystal oscillator.
FIGS. 1A and 1B show the construction of monolithic-type surface-mount crystal oscillators.
In the crystal oscillator shown in FIG. 1A, concavities are formed in both principal surfaces of the receptacle body made of, for example, laminated ceramic, the concavity on one of the principal surfaces being crystal unit housing 1 for housing the crystal unit. Quartz crystal blank 3 which constitutes the crystal unit is contained in crystal unit housing 1 with one end of crystal blank 3 secured by conductive adhesive 4 to the bottom surface of the concavity of crystal unit housing 1. Planar cover 5 is placed so as to cover the concavity and hermetically seal crystal blank 3 inside the concavity. An approximately rectangular concavity which is formed on the other principal surface of the receptacle main body constitutes element housing 2. In element housing 2, approximately rectangular IC chip 6 incorporating components such as an oscillation circuit is secured to the bottom surface of the concavity. In addition, mounting electrodes 9, which are provided for establishing electrical connections when this surface-mount crystal oscillator is mounted on an external wiring board, are provided on areas corresponding to the periphery of the concavity of element housing 2. IC chip 6 and crystal blank 3 are electrically connected by conduction paths (not shown in the figure) which are formed inside the receptacle main body, and further, IC chip 6 and mounting electrodes 9 are also electrically connected by conduction paths which are formed inside the receptacle main body or by circuit patterns which are formed on the surface of the receptacle main body.
The crystal oscillator shown in FIG. 1B is substantially identical to the oscillator shown in FIG. 1A, but differs from the oscillator shown in FIG. 1A in that the concavity for crystal unit housing 1 is not formed on the principal surface of the receptacle main body. Instead, crystal blank 3 is sealed by means of cover 5, in which a concavity of a size which can accommodate crystal blank 3 is formed.
FIGS. 2A and 2B show the construction of bonded-type surface-mount crystal oscillators.
The bonded-type surface-mount crystal oscillator involves first preparing crystal unit 1A in the form of a crystal unit housing in which crystal blank 3 has been sealed, and package substrate 2A having a concavity, IC chip 6 being secured to the bottom surface of this concavity. Package substrate 2A is made of, for example, laminated ceramic. Electrodes 7 which include a pair of terminals used for electrical connection to crystal blank 3 are formed at the four corners of crystal unit 1A. Electrodes 7 are electrically connected to crystal blank 3 by means of a conductive path (not shown in the figure) which passes through the base wall of crystal unit 1A and a circuit pattern which is formed on, for example, the surfaces of the concavity. Joining electrodes 8 are formed on package substrate 2A at positions which correspond to electrodes 7 of crystal unit 1A. The two components, that is, crystal unit 1A and package substrate 2A, are unified by joining electrodes 7 of crystal unit 1A to joining electrodes 8 of package substrate 2A, thereby completing the surface-mount crystal oscillator. At this time, crystal unit 1A may be bonded to the principal surface of package substrate 2A in which the concavity has not been formed, as shown in FIG. 2A, or may be bonded so as to cover the concavity on package substrate 2A as shown in FIG. 2B. These bonded-type surface-mount crystal oscillators are further provided with mounting electrodes 9, which are used to establish electrical connections when the surface-mount crystal oscillator is mounted on an external wiring board, at positions which will be on the outside surface after bonding. Both joining electrodes 8 and mounting electrodes 9 also electrically connect with IC chip 6 by way of the circuit pattern or conductive paths not shown in the figure.
In either of the above-described surface-mount crystal oscillators, IC chip 6 is secured to the bottom surface of the concavity of the element housing by means of, for example, face-down bonding which is realized by ultrasonic thermo-compression bonding using bumps 10. Large-capacitance capacitors 11 which are difficult to integrate in IC chip 6 are loaded together with IC chip 6 on the bottom surface of the concavity of the element housing. In these surface-mount crystal oscillators, moreover, protective resin layer 12 is typically provided on the bottom surface of the concavity of the element housing as generally shown in FIG. 3 to protect the surface layer of IC chip 6 which has exposed terminals. The surface layer of IC chip 6 in this case is a layer which is formed on, of the two principal surfaces of the IC chip, the principal surface which electrically connects to the bottom surface of the element housing in face-down bonding. In particular, when face-down bonding is implemented by using bumps, a gap may occur between the surface layer and the bottom surface of the concavity, and the surface layer therefore tends to come in contact with the outer atmosphere by way of this gap.
Protective resin layer 12 is formed by injecting fluid resin from a nozzle onto the bottom surface of the concavity of the element housing following face-down bonding of IC chip 6, whereby this fluid resin penetrates from the bottom surface side of IC chip 6, following which the resin layer is cured. In this case, protective resin layer 12 preferably covers the entire surface layer of IC chip 6, but as a minimum, protective resin layer 12 must be applied along the outer periphery of IC chip 6 to isolate the surface layer from the outer atmosphere.
With the progress toward greater compactness or miniaturization of surface-mount crystal oscillators in recent years, crystal oscillators having planar outer dimensions measuring, for example, 4 mmxc3x972.5 mm have been developed. In such cases, accommodating both IC chip 6 and large-capacitance capacitors 11 in the element housing is problematic, and capacitors 11 are therefore no longer arranged inside the crystal oscillator but rather, provided by attachment to the external wiring board on which the surface-mount crystal oscillator is mounted. Thus, as shown in FIG. 4, only IC chip 6 is accommodated in element housing 2 to guarantee space for loading IC chip 6.
Nevertheless, the external size of IC chip 6 in a crystal oscillator which has been miniaturized in this way is only slightly smaller than the dimensions of the bottom surface of the concavity of element housing 2. As a result, the gap between IC chip 6 and the inner walls of the concavity thus becomes smaller than the diameter, and particularly the inner diameter, of the nozzle which is used for injecting the fluid resin. Even when an injection needle is used as the nozzle, the inner diameter of the nozzle is on the order of 0.4 mm. In contrast, if the outer planar dimension of the crystal oscillator is 4 mmxc3x972.5 mm, then the dimension of the bottom surface of the concavity will be approximately 2.3 mmxc3x971.6 mm and the planar dimension of IC chip 6 will be approximately 1.9xc3x971.3 mm. If IC chip 6 is secured to the bottom surface of the concavity without any inclination to one side, the gap between the IC chip 6 and the side walls of the concavity will be a maximum of approximately 0.2 mm.
As illustrated in FIG. 5, when the gap between IC chip 6 and the inner walls of the concavity is this narrow, the surface tension and viscosity of fluid resin 12A will impede penetration of fluid resin 12A despite injection of fluid resin 12A from this gap by nozzle 13, and in some cases, an adequate amount of fluid resin 12A will not reach the bottom surface of the concavity. If fluid resin does not penetrate as far as the bottom surface of the concavity, the surface layer of IC chip 6 will come into contact with the outer atmosphere, and this contact will lead to damage to the IC chip due to the humidity content in the atmosphere. This problem can be solved by reducing the size of the IC chip, but in the current state of the art, the above-described outer dimensions must be maintained to guarantee the functionality of the oscillation circuit or the like. Another possible solution would involve reducing the width of the frame portion which is formed along the periphery of the concavity of the element housing to increase the size of the concavity without increasing the outer dimensions of the crystal oscillator, but if the maintenance of mechanical strength or the formation of the mounting electrodes and joining electrodes on this frame portion are taken into consideration, a uniform reduction of the width of the frame portion becomes problematic.
It is an object of the present invention to provide a surface-mount crystal oscillator which allows miniaturization while allowing adequate protection of an IC chip.
The object of the present invention is achieved by means of a surface-mount crystal oscillator which includes a crystal unit housing in which a crystal blank is accommodated and hermetically sealed; and an element housing which is provided on the bottom-surface side of the crystal unit housing, in which a concavity having an open face is formed, and in which an IC chip is secured to the bottom surface of the concavity; wherein a notch portion which is adjoining and connecting to the concavity and which is used for injecting fluid resin is formed in the frame portion of the periphery of the concavity; and wherein a protective resin layer of the IC chip is formed by injecting the fluid resin onto the bottom surface of the concavity by way of the notch portion.
In the present invention, a portion of the side walls of the concavity of the element housing is notched to serve as conduits for injecting fluid resin, thereby facilitating the injection of the fluid resin. As a result, a protective resin layer for protecting the IC chip can be reliably formed, and the IC chip can therefore be reliably protected despite further size reduction and miniaturization of a surface-mount crystal oscillator.